linux-kernel - Re: [PATCH] PCI: Only enable IO window if supported

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Date:	Wed, 24 Jun 2015 09:14:43 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
Cc:	Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
	Guenter Roeck <linux@...ck-us.net>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"suravee.suthikulpanit@....com" <suravee.suthikulpanit@....com>,
	Will Deacon <Will.Deacon@....com>
Subject: Re: [PATCH] PCI: Only enable IO window if supported

On Tue, 2015-06-23 at 18:02 -0500, Bjorn Helgaas wrote:
> On Tue, Jun 23, 2015 at 5:46 PM, Benjamin Herrenschmidt
> <benh@...nel.crashing.org> wrote:
> > On Tue, 2015-06-02 at 15:55 +0100, Lorenzo Pieralisi wrote:
> >> While at it, do you think it is reasonable to also claim the bridge
> >> windows (resources) in the respective pci_read_bridge_* calls ?
> >
> > No, don't claim in read. There's a clear distinction between gathering
> > resources and claiming them, and we need to keep that.
> >
> > Some fixups might happen in between the two for example.
> 
> Are there any existing fixups like that?  Concrete examples would help
> figure out the best way forward.

Not off the top of my mind, it's been a long time since I wrote the
resource claiming stuff in arch/powerpc but it does make me nervous. We
collect resources when probing and we claim in the survey, those have
been historically very distinct steps.

> Most arches call pci_read_bridge_bases() from pcibios_fixup_bus().  I
> think that's a poor place to do it because it's code that normally
> doesn't have to be arch-specific.  Resource claiming is also usually
> done from arch code, and it shouldn't be arch-specific either.

Claiming as in putting in the resource tree etc... is different from
actually reading the values from the HW and is traditionally done much
later, no ?

> If we move both the read and claim into generic code, then we might
> need to make sure there's a fixup phase in between or something.

Well, then there's a more general argument to be made as to whether we
want the claiming to be "merged" as part of the probing/reading I
suppose...

Then there's also the case where everything gets fully reassigned, like
powernv, where the "read" phase is really only about sizing device
BARs...

Cheers,
Ben.


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